Nile tilapia Oreochromis niloticus (4.12±0.05g initial weight) and red seaweed Gracilaria tenuistipitata were co-cultured in 250-L tanks for 8 weeks. Six feeding treatments were conducted, with the control group (monoculture) being fed a commercial pellet (30% protein, 6% lipid) at 100% feed ration (4% of biomass per day), while fish in the five other groups were co-cultured with red seaweed (1 kg m^-3) and received pellet at 80%, 60%, 40%, 20%, and 0% of the feeding ration of the control. The results indicated that the concentrations of TAN, NO₂^-, NO₃^-, and PO₄^3- in the co-culture tanks were much lower than in the monoculture. Moreover, the growth rate and productivity of fish in the 80% feeding ration treatment were comparable to those in the control group (p>0.05). However, fish performance gradually reduced with a decrease in feeding rates from 60% downwards, and all were significantly inferior to the control group (p